Measure twice and cut once. I first learnt this when I fabricated steel that it was not appreciated if you wasted any material.
Using a 1.0 x 0.76 x 3.0mm Foamboard sheet (they come in all thickness and sizes, 10,5 and 3mm and in sizes A2, A3 and A4), I lightly draw out the sides and ends using a 0.5 HB mechanical pencil. Ensure you start from a square corner, most are. Then for cutting out use a new sharp edge angle blade.
This is because you need a clean cut through the paper both sides of the board. Once you have all the visible components cut i.e. the pieces you want to draw detail into, (other structural pieces can be left with the paper on.)
Go boil a kettle (have a cup of tea/coffee?) and place them into a Pyrex dish or a flat bottomed container like a metal tray that won’t shatter.

Once boiled pour the water over the foam board on both on sides. You need the paper layers to surface bubble all over to enable the removal of the outside paper.
After taking the foam board from the dish (avoiding scalding one’s fingers of course) placing them on a flat surface and finally using your fingers, gently rub the paper off from both sides of the piece.
The reason being that this helps avoid later warpage. Cardboard slate tile roofs on foam board are a different challenge; I’ll cover this later on.

Now I’ve seen a few methods shown on You Tube and so forth, but for me the foam board I’m using must have a different quality paper from any shown and likes to retain its covering apparently.
Yours may quite easily peel off after taking a hot shower, mine didn’t!
Don’t be afraid to re soak the board to remove stubborn paper, as its closed cell construction will leave it with a flat non-soggy surface to draw on once it has thoroughly dried off.
Placing the pieces on a flat surface and sandwiching them between a heavy board and/or books on top will help too.

Well I said that these buildings were a challenge and sure enough it arrived in the form of longitudinal shrinkage. The six-terrace house length worked out by yours truly of 570mm had shrunk by about 5mm, which meant that it affected the window/door positions.
Only the length was affected and the 100mm wall width had only a negligible change.
This shrinkage was not a problem in my earlier shorter station buildings and this was a handy misfortune, being that the Dandy Mire Viaduct may suffer from the same issue later on.
The cure was straightforward, add an extra 10mm onto the length and cut back to the correct length afterwards. I suspect that boiling water shrunk the foam cells, but until I find a better method of paper removal I will live with it! The old pieces I saved for other building purposes later.